The long-term objective of this proposal is to study the regulation of inorganic phosphorus (Pi) homeostasis by female sex steroids, mainly Estrogen (E2). Inorganic phosphorus (Pi) is an important mineral that is essential to various important physiological and biochemical processes. Clinical studies have shown that women treated with E2 exhibited renal Pi wasting and hypophosphatemia. Both kidney and intestine play an important role in the control and maintenance of Pi homeostasis. Studies have shown that E2 stimulates Pi absorption in the intestine; however, the effect of E2 on renal handling of Pi is poorly understood. Our preliminary studies showed that E2 treatment of ovariectomized (OVX) rat caused urinary Pi wasting and hypophosphatemia. This effect resulted from a specific downregulation of type IIa apical sodium-dependent phosphate cotransporter (NaPi-IIa), independently of changes in food intake and parathyroid hormone (PTH) levels. In vitro studies indicated that this effect is mediated through a posttranscriptional mechanism involving the mRNA stability of NaPi-IIa. To fully characterize this new regulatory pathway of Pi metabolism, we propose to perform the experiments described in the following two Specific Aims. Aim 1: Characterize the rapid, non-genomic effect of E2 on Pi handling by the kidney. The effects of E2 on urinary Pi excretion, NaPi-IIa transport activity, and its membrane expression (trafficking) will be examined in female OVX rats. The transduction pathways mediating this effect will be investigated using both in vitro and in vivo studies involving PDZK1 and NHERF1 knout mice. These studies will be complemented with in vitro experiments, which will examine the expression and transport activity of NaPi-IIa using proximal tubule suspensions, Xenopus Laevis oocytes and MDCK cells. Aim 2: Determine the molecular mechanisms and transduction pathways mediating the chronic effect of E2 on renal Pi handling in OVX female rats. We will use the pharmacological agonists as well as mice with genetic deletion of estrogen receptors ER? or ER?. The role of mRNA stability and 3'UTR in E2- induced downregulation of NaPi-IIa will be determined using COS7 cells transfected with NaPi-IIa full- length cDNA. These studies will use both animal models as well as cell culture experiments. Once completed, these studies will provide us with a full characterization of E2 as a novel regulatory factor of Pi homeostasis with a clear understanding of its effects on renal handling of Pi. We propose that E2 could be an important and efficient therapeutic tool that can prevent hyperphosphatemia and its adverse effects on cardiovascular system in conditions associated with parathyroid gland dysfunction. PUBLIC HEALTH RELEVANCE: Inorganic phosphorus (Pi) is an important mineral involved in several functions of the body. The blood level of Pi is controlled by estrogen, which increases its excretion in the urine. This effect should be considered when prescribing estrogen, and this hormone can also be used when blood level of Pi is high (hyperphosphatemia).